During conventional polymerization, a marked viscosity increase of the reaction mixture occurs as a result of the high molecular weight produced during the polymerization, in particular of acrylic acid or acrylamide. A viscosity increase of this type gives problems in achieving thorough mixing and difficulties in removing the heat of reaction. Conventional processes therefore require the use of very complicated polymerization reactors if low conversions and low polymer content are to be avoided.
High-molecular-weight polyacrylic acid or polyacrylamide are therefore currently prepared predominantly in systems of more than one phase, for example by suspension polymerization or emulsion polymerization. The advantages of emulsion/suspension polymerization may be summarized as follows:
1.) The viscosity of the dispersion is independent of the degree of polymerization and therefore the heat of polymerization can be dissipated in a controlled manner. PA1 2.) Relatively easy incorporation of components by feed processes. PA1 3.) There is a high steady concentration of polymer in the growing particles. PA1 4.) Copolymerizations generally proceed in a steady manner to give chemically uniform copolymers. PA1 1st step: Preparation of a monomer/water-in-oil emulsion PA1 2nd step: Free-radical polymerization of the monomers in the monomer/water droplet, initiated by redox systems and/or free-radical initiators PA1 3rd step: Addition of an inverting surfactant which ensures that the polymer phase is liberated smoothly in the aqueous medium from the W/O system by inversion via a W/O/W system. PA1 .eta.=viscosity of emulsion PA1 .phi.=phase volume ratio PA1 C.sub.E =emulsifier concentration PA1 A, B=system-specific constants PA1 g=gravitational acceleration constant PA1 .eta..sub.o =viscosity of emulsion PA1 the chemical structure of the emulsifier used PA1 the degree of dispersion PA1 the polarity of the oil phase used. PA1 Fs is an acyl group of a fatty acid or a fatty acid mixture having from 8 to 18 carbon atoms, PA1 where a) and b) are present in a weight of from 100:0 to 60:40. PA1 a) providing a monomer/water-in-oil emulsion using an emulsifier, PA1 b) free-radical polymerization of the monomers in the monomer/water droplets by means of suitable initiator systems and PA1 c) addition of an inverting surfactant to liberate the polymer phase from the water-in-oil system, wherein the emulsifier is a mixture of glycerol monooleate and one or more diacetyltartrates of fatty acid glycerides.
The inverse emulsion polymerization process, furthermore, permits the preparation of easily handleable water-soluble polymers in emulsion form which can be dissolved much more rapidly than dry polymer powder by inverting the polymer-in-oil emulsion with the addition of water.
High-molecular-weight polymers prepared by inverse emulsion polymerization have gained increasing industrial importance in recent years.
Linear high-molecular-weight polymers made by inverse emulsion polymerization are now used almost exclusively in many applications: in papermaking a cationic acrylamide copolymer (Mw&gt;10.sup.7 g/mol) is added to the pulp suspension to bring about microflocculation, which binds the pulp fibers to the fillers and also accelerates the removal of water.
In water treatment, agglomerations of polymer form bridges between a number of suspension particles, creating relatively large flocs which accelerate sedimentation and the filtration of finely divided suspensions of solids. Above a molar mass of about 2.times.10.sup.6 g/mol, the rate of sedimentation rises in proportion with the mass of the polymer.
Stabilization of emulsions and dispersions.
An inverse polyacrylic acid or polyacrylamide emulsion is usually prepared in three steps.
Emulsifier mixtures for use in the inverse emulsion polymerization of crosslinked polymers, in particular polyacrylic acids, are known, for example, from U.S. Pat. No. 5,216,070. This document proposes a process for forming water-in-oil emulsions which gives a water-soluble polymer by emulsion polymerization of corresponding monomers in the presence of an initiator and of an emulsifier. The emulsifier used is a polysiloxane-polyalkylene-polyether copolymer. The polymerization of the acid monomer can be carried out with partial or complete neutralization of the monomer and without adverse effects on the stability of the emulsion. Alkoxylated copolymers of this type which are used as emulsifier mixtures, however, frequently cause problems with biodegradable emulsifier systems, in particular based on renewable raw materials.